This competitive renewal application will advance the development of an intravascular bi-modal technology for diagnosis of arterial wall pathologies including rupture-prone (vulnerable) atherosclerotic plaques. We propose to optimize, construct and test a unique clinically-compatible system that combines fast, time-resolved fluorescence spectroscopy (TRFS) to dynamically evaluate atherosclerotic plaque composition under pull-back motion, with intravascular ultrasound (IVUS) that allows for both visual reconstruction of plaque microanatomy and guidance of TRFS measurements. The resulting system will enable detection and monitoring of biochemical, functional and structural features of atherosclerotic lesions with clinical relevance (e.g. predictive of plaque rupture). In this renewal application, we propose to advance the integration and in-vivo validation of this bi-modal technology and prepare for clinical intravascular evaluation through the following specific aims: Aim 1. To design, construct and optimize prototype bi-modal (TRFS-IVUS) intravascular catheters to demonstrate (1) the technical feasibility of integrating the TRFS with single element transducer IVUS catheters and (2) the ability of the bi-modal system to provide real-time diagnostic feedback information concerning arterial wall composition and structure. To achieve this we will build two catheter systems and validate their technical performance in-vitro (tissue phantoms, arterial segments). Aim 2. To demonstrate in-vivo the validity of continuous/radial TRFS data acquisition under pull-back motion and under IVUS guidance. To achieve this we will conduct transluminal procedures in an atherosclerotic pig model using Catheter Assembly I. We will determine optimal experimental parameters for dynamic TRFS acquisition in pulsatile blood flow conditions, evaluate the limiting design factors for the bi-modal catheter, and determine design and experimental parameters to optimize co-registration of TRFS and IVUS data. Aim 3. To determine the ability of optimized Catheter Assembly II to operate intravascularly in various arterial beds, including coronary arteries, and to determine its diagnostic capability. This will be achieved by testing the bi-modal technique in an atherosclerotic pig model (in-vivo) and in human coronary segments (ex-vivo). This will demonstrate the feasibility of the catheter prototype to operate effectively intravascularly under conditions of blood flow and motion, to collect co-registered TRFS/IVUS, and to generate diagnostic information. Aim 4. Establish the feasibility of TRFS-IVUS to dynamically and in near-real time (few seconds) characterize, discriminate and visualize relevant intravascular pathologies. To achieve this we will develop computational/classification models employing features derived from TRFS-data, IVUS RF-data (virtual histology) and IVUS greyscale (echogenicity) images; apply these models to data derived from bi-modal measurements (Aims 2 & 3) to determine the sensitivity, specificity, and overall predictive value of the proposed method; and validate this data against tissue histopathology. Aim 5. Prepare and submit an application for an FDA Sponsor-Investigator Investigational Device Exemption (IDE) for future clinical evaluation of the bi-modal system. This will make use of experimental data and results obtained in Aim 3 and Aim 4 and additional tests for evaluation of safety, effectiveness, and diagnostic capabilities as required by the FDA.